Nanocrystals could help recover waste heat

Researchers in the US have come up with a new and simple way to boost the performance of a common thermoelectric material — bismuth antimony telluride — which had defied attempts at improvement for over 50 years.

Thermoelectric materials convert heat directly into electricity and could be used to boost the efficiency of conventional coal and nuclear power stations by recovering heat that is normally wasted. The materials could also improve the effectiveness of solar cells and could be used to cool computer chips and other devices.

To be used in such ways, however, a thermoelectric material must be good at conducting electricity but poor at conducting heat. This requirement is expressed in the thermoelectric figure of merit ZT, which should be greater than 1.0. The ZT of bismuth antimony telluride — one of the most common thermoelectric materials — has remained stubbornly at around 1.0 for more than half a century.

Now, Zhifeng Ren and Gang Chen and colleagues at Boston College and the Massachusetts Institute of Technology have made a significant breakthrough by milling the material into a fine powder that contained nanoparticles measuring about 20 nm across (Sciencexpress DOI: 10.1126/science.1156446). Next, they hot-pressed the powder into nanocrystalline ingots.

Improved ZT

The researchers found that the thermoelectric figure of merit (ZT) for the ingots increased to 1.2 at room temperature (from a value of 1.0 previously). Moreover, they found that the ZT peaks at 1.4 at 100 °C.

This might not sound like a big improvement, but the researchers describe it as a “significant step” towards creating materials that are useful for cooling and power generation.

Electrical transport measurements on the ingots, together with microstructure and modelling, showed that the ZT improves thanks to the low thermal conductivity caused by increased phonon scattering at grain boundaries and defects in the material. However, the electrical conductivity of the material is not affected signficantly by grain boundaries and defects.

Promising for applications

The high ZT in the temperature range 25–250 °C makes these materials promising for cooling and waste heat recovery applications, say the researchers. Potential applications include converting the heat of car exhausts into electricity, for example. “Other applications include efficient thermoelectric cooling, such as air conditioning and refrigeration, and solar thermoelectricity,” explained Ren and Chen.

The researchers have also built a prototype cooling device to confirm the properties.The team now plans to make efficient coolers and power generators using the improved materials. “At the same time, we will apply the approach to other promising thermoelectric materials,” they stated.

The company GMZ Energy Inc, a Massachusetts-based start-up, is now mass-producing the materials.

Cadmium Mercury Telluride.

I wonder if similar results could be obtained from other thermoelectric compounds? Cd.Hg.Te for instance. Peltier Effect heat pumps have the advantage of being small. Preservation of medical specimens during transport in hot climates could possibly benefit from this good news? Seebeck thermoelectric efficiency will still remain in the single percentage digits,(Carnot limitations) but where waste heat is a free by-product, efficiency is less important.